1. Field of the Invention
The invention relates to a method of switching on an inductive load, in particular an ignition coil, whose current is intended repeatedly to reach an end value at a specific time.
2. Description of the Related Art
To reach a predefined current at a specific predefined time following the application of a voltage to an inductive load, the action of switching on the load is required to occur at a first time before the predefined time, the first time depending on the slope of the current rise. The slop of the current rise in turn depends on the inductance, the battery voltage, contact resistances and the temperature.
The object of the present invention is to perform the action of switching on the inductive load such that a current conducted by the load reaches a predefined end value at a predefined time. More specifically, in the electronic ignition of internal combustion engines, a predefined ignition energy is to be ensured which, if necessary, may be varied considerably as a function of operating parameters of the internal combustion engine, just like the ignition time.
According to the present invention, this object is achieved by measuring the time interval between the switching-on of the inductive load and reaching at least one intermediate current value. The measured time interval and the at least one predefined intermediate value are used to calculate the anticipated time interval from switching on the inductive load until the end value is reached. A following switching-on action is carried out at the calculated time interval before the respectively predefined time.
The curve associated with current rise in inductive loads is not a straight line, but has an individual curvature which depends on various influencing variables, such as battery voltage, contact resistances in the cabling and the connectors, resistance changes arising from temperature or aging. Depending on the design, these influencing variables can be controlled out individually by the method according to the invention.
In an embodiment of the present invention, the time is calculated using a function representing the current rise when a substantially constant voltage is applied. The function is preferably stored in a memory. This embodiment allows registration of a current curve which varies with respect to its slope.
To account for changes in the curvature of the current curve, the method according to the invention may be designed such that the function is calculated from a plurality of predefined intermediate values and the times associated with these.
In a further embodiment of the present invention, the measured time interval is used to calculate at least one parameter of a predefined function. In the following switching-on action, the calculated time is determined by using the function, the at least one parameter and the end value.
In this embodiment, provision is preferably made for the function to be i=î(1xe2x88x92exe2x88x92t.R/L), i is the current at the time t, î is the current reached at infinity, R is the resistance and L is the inductance, and the parameter calculated to be R/L. For example, to determine R/L the inductive load is switched on and the time to reach an intermediate current value is measured. The time to reach the intermediate current value is inserted in place of t in the equation and the intermediate current value is i. The value î is measured by measuring the current through the inductive load after a steady state has been reached. From these values, R/L is then calculated in the above equation. The calculated R/L and the desired end value as i may then be used to determine the time duration between the switching-on action and reaching the end value.
The method according to the present invention is preferably performed in the processor of a control device. To avoid complicated calculations, the function may be stored as a table, in each case a time until the predefined intermediate value is reached being assigned to the associated time between the switching-on action and reaching the predefined value.
In yet another embodiment of the present invention, the time between the switching-on action and the anticipated reaching of the end value is calculated in accordance with the rule of three, using a correction value which represents the curvature of the function.
If a change in the curvature of the curve should be negligible in an application, it is further possible, within the scope of the method according to the present invention, for the time between the switching-on action and the anticipated reaching of the end value to be calculated in accordance with the rule of three, i.e., a method of finding a fourth number from three known numbers, of which the first has the same proportion to the second as the third does to the fourth.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.